Dynamically simulated tropical storms in a changing climate and their impact on the assessment of future climate risk Ray Bell Supervisors – Prof. Pier Luigi Vidale, Dr. Jane Strachan and Dr. Kevin Hodges Tropical Hour – 11th Jul

About me Previous studies Models Tracking algorithm Results Future work Tropical cyclones and climate change

MOcean – NOCS Met office placement 2010 About me Wind and Wave variability in the North Atlantic Ocean wave ensemble forecasting Air-Sea interactions in TCs?

Impacts on the climate system, removing heat and moisture from the ocean affecting large scale circulation Socio-economic impacts Associated risk with climate change Katrina Current damage $91.5bn (ICAT, 2011) Motivation

Will there be a change in TC activity (location, intensity, frequency, lifetime) with climate change? How well are the governing mechanisms represented in climate models and how does resolution affect their representation? How do TCs change in a warmer world with increasing resolution? What is the role of teleconnections - Possible ENSO changes on NAtl TC activity? [What is the role of coupling with the ocean?] PhD questions

Uncertainties in future dynamical and thermodynamical changes due to anthropogenic warming in the tropics. Change in precursors (e.g. AEWs...) Current trends and attribution Natural variability (Pielke et al, 2005) vs. Climate change (Webster et al, 2005) Limitations in the quality and availability of data (Kossin et al, 2007) Theory (Emanuel, 1987) and high resolution modelling studies (Knutson et al. 2004; Oouchie et al. 2006) predict TCs will become more intense. Little theory explaining the change in TC frequency - likely to become less frequent in a warmer world. Previous studies

Global increase in TC intensities of 2-11% (surface wind speed). (3-21% decrease in central pressure) by 2100 (A1B scenario) (Knutson et al, 2010) Decrease in globally averaged frequency of TCs by 6-34% - Weakening of the tropical circulation (Increase in dry static stability) (Sugi, 2002) - Increase in q* causes larger X m (moist S in middle of troposphere and that of the BL), therefore takes longer to moisten mid-trop where intensification can occur (Emanuel, 2008) More robust decrease in the SH – smaller increase in SST and increase wind shear (Vecchi, 2007; Zhao et al, 2009) Increase of 20% in precipitation rate within 100km of the storms centre (Knutson et al, 2010) Higher resolution models needed to pick up a warming related intensification (<60km) (Bengtsson et al, 2007) Previous studies

Previous studies (Knutson et al, 2010) Large Regional Uncertainties

Models NH May - Nov HadGAM -135km HiGEM - 90km NUGAM - 60km

T42 ξ 850 (0 o -60 o N)– Reduce noise. Comparison of different spatial resolution data Minimum lifetime of 2 days and no constraint on the minimum displacement distance. Capture more of TC lifecycle Cyclogenesis (0-20 o N over land, 0-30 o N over ocean) Coherent vertical structure and warm core Max T63 vor at each level from 850hPa to 250hPa Intensity threshold T63 ξ 850 > 6x10 -5 s -1, ξ 850 – ξ 200 > 6x10 -5 s -1, for at least 1 day (4 x 6hr) Associate full res data with tracks Wind speed must attain 20m/s at 850hPa (change in slightly more intense TCs) Statistical packages TRACK Hodges (1995); Bengstsson et al. (2007)

Jane’s previous work

Lack of re-curving in NWPAC - SSTs in the NWPAC? HiGEM CTRL TC activity

HiGEM CTRL TC frequency Reduced ATL activity – Cold bias? - ATL known to the most sensitive basin to SST (Emanuel, 2008) Max intensity ~cat 3 (50m/s +) Obs from Using IBTrACS (not same criteria as tracking) * att is attain 850hPa wind speed >20m/s dataset

My results - Change in track densities

Change in frequency ATL shows largest decrease EPAC shows an increase at 2xCO 2 NIND shows an increase - attain 850hPa wind speed 20m/s gives a + change in TC freq. Suggesting largest decrease of TCs from weaker storms * att is attain 850hPa wind speed >20m/s dataset

Change in intensity ξ 850

Change in intensity (850hPa wspeed)

Regional change of 850hPa wspeed

Change in duration 30 years

Change in structure 100 most intense TCs at most intense 4xCO2 Earth relative winds 2xCO2 10 o radius

Future work Attribution of change in TC activity - SST – Local vs. Remote changes in each basin - Wind shear - Change in other parameters? (dry static stability, low level vor, RH, genesis parameters) Changes in SH ENSO influence on NATL activity (IBTrACS, re-an, ctrl, future) Model HiGEM SSTs on HadGAM (TCs in other model resolutions)

References Bengtsson, L., Hodges, K. I., and Esch, M. (2007). Tropical cyclones in a T159 resolution global climate model: comparison with observations and re-analysis. Tellus A, 59, 396–416. Bengtsson, L., Hodges, K. I., Esch, M., Keenlyside, N., Kornblueh, L., Luo, J-J., Yamagata, T. (2007) How may Tropical cyclones change in a warmer climate. Tellus A, 59, Emanuel, K. A. (1987) The dependence of hurricane intensity on climate. Nature, 326, Hodges, K.I. (1995) Feature tracking on a unit sphere Mon. Weath. Rev. 123, pp Knutson, T. R., McBridge, J. L., Chan, J., Emanuel, K., Holland, G., Landsea, C., Held, I., Kossin, J. P., Srivastava, A. K., and Sugi, M. (2010). Tropical cyclones and climate change. Nat. Geosci., 3, 157–163. Knutson, T.R., and Tuleya, R.E. (2004) Impact of CO2-induced warming on simulated hurricaned intensity and precipitation: sensitivity to the choice of climate model and convective parameterization. J. Climate Kossin, J.P., Knapp, K. R., Vimont, D.J. Murnane, R. J., Harper, B.A. (2007) A globally consistent reanalysis of hurricane variability and trends. Bull. Amer. Met. Soc. 88, McDonald, R., Bleaken, D., Creswell, D., Pop e, V., and Senior, C. (2005). Tropical storms: representation and diagnosis in climate models and the impacts of climate change. J. Climate, 18, 1275–1262. Oouchie, K., Yoshimura, J., Yoshimura, H., Mizuta, R., Kusunoki, S. And Noda, A. (2006) Tropical cyclone climatology in a global-warming climate as simulated in a 20-km mesh global atmospheric model: frequency and wind intensity analysis. J. Met. Soc. Jap. 84, Pielke, R. A. J., Landsea, C., Mayfield, M., Laver, J., and Pasch, R. (2005). Hurricanes and global warming. Bul l. Amer. Meteorol. Soc., 86, 1571–1575. Webster, P., Holland, G., Curry, J., and Chang, H.-R. (2005). Changes in tropical cyclone number, duration, and intensity in a warming environment. Science, 309, 1844–1846. Questions/Comments/Suggestions ?

References Sugi, M. Noda, A., Sato, N. (2002) Influence of global warming on tropical cyclone climatology: an experiment with the JMA global model. J. Met. Soc. Jap. 80, Vecchi, G. A. And Soben, B. J. (2007) Increased tropical Atlantic wind shear in model projections of global warming. Geophys. Res. Let. 34, L08702 Zhao, M., Held, I. M., Lin, S-J. And Vecchi, G. A. (2009) Simulations of Global Hurricane Climatology, Interannual Variability, and Respone to Global Warming using a 50-km Resolution GCM. J. Climate Questions/Comments/Suggestions ?

Stong upward trends of short lived ( 2 days (Landsea et al. 2009) – possible changes in obs. Procedure. - Downscalling studies suggest no change in duration Emanuel (2008) Some potential for the poleward movement of TC to ETC transition but lack of studies Possible Increase in genesis in central ocean basins, especially Central Pacific (Li, 2010) Yoshimura and Sugi (2005) did experiments changing SST and CO2. Found SST had little affect and CO2 had large affects. Many studies find dynamic factors (low level vorticity and vertical wind shear) play a more important role than thermodynamic factors (SST and moist instability). Previous studies

Change in genesis densities

Change in lysis densities

Future work Track transient data (Last 30 years – First 30 years) Comparisons to CTRL and 2xCO 2, 4xCO 2 CMIP5 models if res is high enough HiGEM SSTs on HiGAM – Lead to More intense/More frequent TCs? Ocean coupling on different time scales? - CTRL and CO 2 inc runs?